=head1 NAME
X X
perldebug - Perl debugging
=head1 DESCRIPTION
First of all, have you tried using the B switch?
If you're new to the Perl debugger, you may prefer to read
L, which is a tutorial introduction to the debugger .
=head1 The Perl Debugger
If you invoke Perl with the B switch, your script runs under the
Perl source debugger. This works like an interactive Perl
environment, prompting for debugger commands that let you examine
source code, set breakpoints, get stack backtraces, change the values of
variables, etc. This is so convenient that you often fire up
the debugger all by itself just to test out Perl constructs
interactively to see what they do. For example:
X
$ perl -d -e 42
In Perl, the debugger is not a separate program the way it usually is in the
typical compiled environment. Instead, the B flag tells the compiler
to insert source information into the parse trees it's about to hand off
to the interpreter. That means your code must first compile correctly
for the debugger to work on it. Then when the interpreter starts up, it
preloads a special Perl library file containing the debugger.
The program will halt I the first run-time executable
statement (but see below regarding compile-time statements) and ask you
to enter a debugger command. Contrary to popular expectations, whenever
the debugger halts and shows you a line of code, it always displays the
line it's I to execute, rather than the one it has just executed.
Any command not recognized by the debugger is directly executed
(C'd) as Perl code in the current package. (The debugger
uses the DB package for keeping its own state information.)
Note that the said C is bound by an implicit scope. As a
result any newly introduced lexical variable or any modified
capture buffer content is lost after the eval. The debugger is a
nice environment to learn Perl, but if you interactively experiment using
material which should be in the same scope, stuff it in one line.
For any text entered at the debugger prompt, leading and trailing whitespace
is first stripped before further processing. If a debugger command
coincides with some function in your own program, merely precede the
function with something that doesn't look like a debugger command, such
as a leading C or perhaps a C, or by wrapping it with parentheses
or braces.
=head2 Calling the debugger
There are several ways to call the debugger:
=over 4
=item perl -d program_name
On the given program identified by C.
=item perl -d -e 0
Interactively supply an arbitrary C using C.
=item perl -d:Ptkdb program_name
Debug a given program via the C GUI.
=item perl -dt threaded_program_name
Debug a given program using threads (experimental).
=back
=head2 Debugger Commands
The interactive debugger understands the following commands:
=over 12
=item h
X
Prints out a summary help message
=item h [command]
Prints out a help message for the given debugger command.
=item h h
The special argument of C produces the entire help page, which is quite long.
If the output of the C command (or any command, for that matter) scrolls
past your screen, precede the command with a leading pipe symbol so
that it's run through your pager, as in
DB> |h h
You may change the pager which is used via C command.
=item p expr
X
Same as C in the current package. In particular,
because this is just Perl's own C function, this means that nested
data structures and objects are not dumped, unlike with the C command.
The C filehandle is opened to F, regardless of
where STDOUT may be redirected to.
=item x [maxdepth] expr
X
Evaluates its expression in list context and dumps out the result in a
pretty-printed fashion. Nested data structures are printed out
recursively, unlike the real C function in Perl. When dumping
hashes, you'll probably prefer 'x \%h' rather than 'x %h'.
See L if you'd like to do this yourself.
The output format is governed by multiple options described under
L.
If the C is included, it must be a numeral I; the value is
dumped only I levels deep, as if the C option had been
temporarily set to I.
=item V [pkg [vars]]
X
Display all (or some) variables in package (defaulting to C)
using a data pretty-printer (hashes show their keys and values so
you see what's what, control characters are made printable, etc.).
Make sure you don't put the type specifier (like C) there, just
the symbol names, like this:
V DB filename line
Use C and C for positive and negative regexes.
This is similar to calling the C command on each applicable var.
=item X [vars]
X
Same as C.
=item y [level [vars]]
X
Display all (or some) lexical variables (mnemonic: C variables)
in the current scope or I scopes higher. You can limit the
variables that you see with I which works exactly as it does
for the C and C commands. Requires the C module
version 0.08 or higher; will warn if this isn't installed. Output
is pretty-printed in the same style as for C and the format is
controlled by the same options.
=item T
X X X
Produce a stack backtrace. See below for details on its output.
=item s [expr]
X X
Single step. Executes until the beginning of another
statement, descending into subroutine calls. If an expression is
supplied that includes function calls, it too will be single-stepped.
=item n [expr]
X
Next. Executes over subroutine calls, until the beginning
of the next statement. If an expression is supplied that includes
function calls, those functions will be executed with stops before
each statement.
=item r
X
Continue until the return from the current subroutine.
Dump the return value if the C option is set (default).
=item
Repeat last C or C command.
=item c [line|sub]
X
Continue, optionally inserting a one-time-only breakpoint
at the specified line or subroutine.
=item l
X
List next window of lines.
=item l min+incr
List C lines starting at C.
=item l min-max
List lines C through C. C is synonymous to C.
=item l line
List a single line.
=item l subname
List first window of lines from subroutine. I may
be a variable that contains a code reference.
=item -
X
List previous window of lines.
=item v [line]
X
View a few lines of code around the current line.
=item .
X
Return the internal debugger pointer to the line last
executed, and print out that line.
=item f filename
X
Switch to viewing a different file or C statement. If I
is not a full pathname found in the values of %INC, it is considered
a regex.
Ced strings (when accessible) are considered to be filenames:
C and C access the body of the 7th Ced string
(in the order of execution). The bodies of the currently executed C
and of Ced strings that define subroutines are saved and thus
accessible.
=item /pattern/
Search forwards for pattern (a Perl regex); final / is optional.
The search is case-insensitive by default.
=item ?pattern?
Search backwards for pattern; final ? is optional.
The search is case-insensitive by default.
=item L [abw]
X
List (default all) actions, breakpoints and watch expressions
=item S [[!]regex]
X
List subroutine names [not] matching the regex.
=item t
X
Toggle trace mode (see also the C option).
=item t expr
X
Trace through execution of C.
See L for examples.
=item b
X
X
Sets breakpoint on current line
=item b [line] [condition]
X
X
Set a breakpoint before the given line. If a condition
is specified, it's evaluated each time the statement is reached: a
breakpoint is taken only if the condition is true. Breakpoints may
only be set on lines that begin an executable statement. Conditions
don't use C:
b 237 $x > 30
b 237 ++$count237 < 11
b 33 /pattern/i
=item b subname [condition]
X
X
Set a breakpoint before the first line of the named subroutine. I may
be a variable containing a code reference (in this case I
is not supported).
=item b postpone subname [condition]
X
X
Set a breakpoint at first line of subroutine after it is compiled.
=item b load filename
X
X
Set a breakpoint before the first executed line of the I,
which should be a full pathname found amongst the %INC values.
=item b compile subname
X
X
Sets a breakpoint before the first statement executed after the specified
subroutine is compiled.
=item B line
X
X
Delete a breakpoint from the specified I.
=item B *
X
X
Delete all installed breakpoints.
=item a [line] command
X
Set an action to be done before the line is executed. If I is
omitted, set an action on the line about to be executed.
The sequence of steps taken by the debugger is
1. check for a breakpoint at this line
2. print the line if necessary (tracing)
3. do any actions associated with that line
4. prompt user if at a breakpoint or in single-step
5. evaluate line
For example, this will print out $foo every time line
53 is passed:
a 53 print "DB FOUND $foo\n"
=item A line
X
Delete an action from the specified line.
=item A *
X
Delete all installed actions.
=item w expr
X
Add a global watch-expression. Whenever a watched global changes the
debugger will stop and display the old and new values.
=item W expr
X
Delete watch-expression
=item W *
X
Delete all watch-expressions.
=item o
X
Display all options
=item o booloption ...
X
Set each listed Boolean option to the value C<1>.
=item o anyoption? ...
X
Print out the value of one or more options.
=item o option=value ...
X
Set the value of one or more options. If the value has internal
whitespace, it should be quoted. For example, you could set C to call B with those specific options.
You may use either single or double quotes, but if you do, you must
escape any embedded instances of same sort of quote you began with,
as well as any escaping any escapes that immediately precede that
quote but which are not meant to escape the quote itself. In other
words, you follow single-quoting rules irrespective of the quote;
eg: C or C.
For historical reasons, the C<=value> is optional, but defaults to
1 only where it is safe to do so--that is, mostly for Boolean
options. It is always better to assign a specific value using C<=>.
The C

can be abbreviated, but for clarity probably should
not be. Several options can be set together. See L
for a list of these.
=item < ?
X<< debugger command, < >>
List out all pre-prompt Perl command actions.
=item < [ command ]
X<< debugger command, < >>
Set an action (Perl command) to happen before every debugger prompt.
A multi-line command may be entered by backslashing the newlines.
=item < *
X<< debugger command, < >>
Delete all pre-prompt Perl command actions.
=item << command
X<< debugger command, << >>
Add an action (Perl command) to happen before every debugger prompt.
A multi-line command may be entered by backwhacking the newlines.
=item > ?
X<< debugger command, > >>
List out post-prompt Perl command actions.
=item > command
X<< debugger command, > >>
Set an action (Perl command) to happen after the prompt when you've
just given a command to return to executing the script. A multi-line
command may be entered by backslashing the newlines (we bet you
couldn't have guessed this by now).
=item > *
X<< debugger command, > >>
Delete all post-prompt Perl command actions.
=item >> command
X<<< debugger command, >> >>>
Adds an action (Perl command) to happen after the prompt when you've
just given a command to return to executing the script. A multi-line
command may be entered by backslashing the newlines.
=item { ?
X
List out pre-prompt debugger commands.
=item { [ command ]
Set an action (debugger command) to happen before every debugger prompt.
A multi-line command may be entered in the customary fashion.
Because this command is in some senses new, a warning is issued if
you appear to have accidentally entered a block instead. If that's
what you mean to do, write it as with C or even
C.
=item { *
X
Delete all pre-prompt debugger commands.
=item {{ command
X
Add an action (debugger command) to happen before every debugger prompt.
A multi-line command may be entered, if you can guess how: see above.
=item ! number
X
Redo a previous command (defaults to the previous command).
=item ! -number
X
Redo number'th previous command.
=item ! pattern
X
Redo last command that started with pattern.
See C, too.
=item !! cmd
X
Run cmd in a subprocess (reads from DB::IN, writes to DB::OUT) See
C, also. Note that the user's current shell (well,
their C variable) will be used, which can interfere
with proper interpretation of exit status or signal and coredump
information.
=item source file
X
Read and execute debugger commands from I.
I may itself contain C commands.
=item H -number
X
Display last n commands. Only commands longer than one character are
listed. If I is omitted, list them all.
=item q or ^D
X
X
Quit. ("quit" doesn't work for this, unless you've made an alias)
This is the only supported way to exit the debugger, though typing
C twice might work.
Set the C option to 0 if you want to be able to step
off the end the script. You may also need to set $finished to 0
if you want to step through global destruction.
=item R
X
Restart the debugger by Cing a new session. We try to maintain
your history across this, but internal settings and command-line options
may be lost.
The following setting are currently preserved: history, breakpoints,
actions, debugger options, and the Perl command-line
options B, B, and B.
=item |dbcmd
X
Run the debugger command, piping DB::OUT into your current pager.
=item ||dbcmd
X
Same as C but DB::OUT is temporarily C